Healthy Skepticism Library item: 13073
Warning: This library includes all items relevant to health product marketing that we are aware of regardless of quality. Often we do not agree with all or part of the contents.
Publication type: news
Brandes L.
Do some vitamins actually increase cancer risk?
Med News Express, CTV.ca 2008 Mar 4
http://healthblog.ctv.ca/blog/_archives/2008/3/4/3558651.html
Full text:
“Is it okay for me to take these?” is a common question my patients ask in the cancer clinic. “These” refer to the contents in the multiple jars they have just stacked on the desk for me to examine.
Not content to simply take a daily low-dose multivitamin (no problem with that as far as I am concerned), most have paid a tidy sum of money for combinations of high-dose vitamins, various other antioxidants, minerals and herbal products, all advertised to boost the immune system and fight cancer. After all, if a little bit of something is good for you, isn’t more even better?
To their consternation, based on the surprising results of several large clinical studies, it is becoming increasingly easy for me to say, “No”.
For example, in a study of over 77,000 people just published in the American Journal of Respiratory and Critical Care Medicine, a significant association between the use of supplemental vitamin E and lung cancer in smokers was observed. According to the study’s lead researcher, Dr. Christopher Slatore, a 7% rise occurred for every 100 mg of the vitamin consumed daily. This translates into a 28% increased risk of lung cancer when 400 mg/day (the dose of vitamin E capsules sold in most stores) is taken for 10 years.
Unfortunately, this is not the only study linking a vitamin with an increased risk of cancer.
A widely-reported 2007 paper in the Journal of the National Cancer Institute found that, compared to non-users of vitamins, men who consumed multivitamins more than once daily, and especially those who took additional vitamin E, beta-carotene (converted to vitamin A in the body) or selenium, had a 32% increased risk of developing advanced prostate cancer and a 98% higher risk of dying from the disease. The added risk of multivitamins was highest in men with a family history of the disease.
The study also found a statistically significant increased risk of localized prostate cancer among heavy multivitamin users who took additional selenium, beta-carotene, or zinc supplements, or who had a positive family history of prostate cancer.
In addition to the prostate cancer findings, other researchers have reported that taking selenium daily increases the risk of recurrent non-melanoma skin cancer.
These studies appear to fly in the face of several previous reports suggesting a protective role of selenium and Vitamin E against cancer.
Indeed, a large Phase 3 clinical trial, called the Selenium and Vitamin E Cancer Prevention Trial (SELECT), involving 35,000 American and Canadian men, is currently studying whether these substances, alone or in combination, can decrease the incidence of prostate cancer!
What are we to believe? Can vitamins and supplements be both pro- and anti-cancer? The answer is almost certainly “yes”.
On the one hand, beta-carotene has been shown in many studies to protect against experimental cancers in the test tube and in rodents. A diet rich in beta-carotene-containing foods is believed to be beneficial and, indeed, essential, to human health. On the other hand, not one, but two large lung cancer prevention trials in smokers have shown that beta-carotene supplements significantly increase the risk of developing lung cancer, prompting the U.S. National Cancer Institute to declare on its website, “Beta-Carotene Supplements Confirmed as Harmful to Those at Risk for Lung Cancer”.
How can this be explained?
Perhaps substances such as beta-carotene, selenium and vitamin E, naturally occurring in food, or even supplemented in moderation as part of a healthy lifestyle (which includes not smoking!) can help decrease the risk of developing certain cancers over a lifetime. However, especially if consumed in higher than normal quantities by smokers with latent (undetected) cancer or, perhaps, by people at genetically high risk for developing prostate cancer, those same substances appear to be able to promote malignant growth.
Finally, what about Vitamin D, currently in fashion to prevent cancer, especially in northern latitudes like Canada, where sun exposure is minimal during the long winter?
While the relatively small studies to date appear promising, based on the emerging story of certain increased cancer risks with the other two fat-soluble vitamins (E and A), I wonder if bad news about vitamin D may not yet trickle in.
Time will tell. In the meantime, the balance of clinical evidence suggests that over-consumption of vitamins should be avoided.
As we say in medicine: “First, do no harm”.
